Understanding AHS Heat Zones Versus USDA Hardiness Zones

What are AHS zones (heat zones) as opposed to USDA Cold Hardiness Zones?

Answer: Many gardeners have heard of or have read about USDA (United States Department of Agriculture) Zones when scanning the aisles of garden centers, reading articles on the latest plants or when deciding what plants to add into their garden. However, AHS (American Horticulture Society) Plant Heat Zones are also imperative to the gardener, though their importance can sometimes be overlooked.

While USDA Zones help measure the cold our plants can endure—that is, if they will survive during the local winter—AHS Heat Zones help to measure the “heat tolerance” of our plants. So to answer your question simply, AHS Zones track heat patterns while USDA Zones track cold hardiness.

USDA Zones are divided into sections based on the average annual minimum temperature with zones ranging from Zone 1a (-60 to -50 degrees F) to Zone 13b (65 to 70 degrees F). They are divided into 10 degree F zones that are then split into two sections of 5 degrees (a and b). Example: Zone 2a (the annual minimum temperature is -50 to -45 degrees F) and Zone 2b (the annual minimum is -45 to -40 degrees F). A plant rated as cold hardy in Zones 4 through 9, for instance, can thrive despite the winters in those zones.

AHS Heat Zones have the same concept with a zone range from Zone 1—areas with fewer than one “heat day” per year—to Zone 12 , which are locations that have more than 210 heat days each year. A heat day is a day with temps about 86F. A plant rated to Heat Zones 6–9, for example, can take the heat of summer in those zones, but would suffer in summers colder or hotter.

With the rising concern of increasing temperatures, AHS Plant Heat Zones are becoming equally as important to gardeners as USDA Hardiness Zones. With the help of these two handy maps, you can create a garden full of beautiful and thriving plants all year long.